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Exposure Reciprocity



There are two competing variables in making a properly exposed negative (actually three I guess if you count film speed.) But if you're just getting started with film photography you may have heard of reciprocity as it pertains to exposure.

By the way reciprocity should not be mistaken for a shortening of reciprocity failure as I often hear people mis-speak.

Reciprocity is the relationship between the length of time of an exposure and the light coming in through the lens controlled by the aperture. In theory, (and usually in practice) doubling the time of an exposure and halfing the light coming in will result the exact same density negative or transparency.

For example, if you meter (or using the "Sunny 16" rule) an exposure as say f16 at 1/125th, your negs would be exposed* exactly the same as if you used:


f32 @ 1/30th
f22 @ 1/60th
f16 @ 1/125th
f11 @ 1/250th
f8 @ 1/500th
f5.6 @ 1/1000th

Note I said exposed exactly the same, not look exactly the same. Though the density of your photo should look exactly the same, the smaller apertures will result in greater depth of field (objects closer and further from the area where you focused will remain in focus.) With a larger aperture the depth of field is shallower (the area near and far from the area where you focused appears less in focus.)

Another thing for photo newbies to note: the way aperture values are marked is somewhat counter intuitive. As the aperture size increases, the number gets smaller. Lenses are usually described by their focal length and their maximum aperture. So a standard old-skool lens for 35mm film is the 50mm lens, and probably the most common maximum aperture is f2. What the f2 is describing is when the aperture is all the way open, the hole measures half of the focal length: 1/2 of 50mm = 25mm.  As you click down the scale on a 50mm lens, the aperture (the size of the hole of the iris) measures:

f2 (1/2 of 50mm) = 25mm
f2.8 (1/2.8 of 50mm) = 17.85mm
f4 (1/4 of 50mm) = 12.5mm
f5.6 (1/5.6 of 50mm) = 8.93mm
f8 (1/8 of 50mm) = 6.25mm
f11 (1/11 of 50mm) = 4.55mm
f16 (1/16 of 50mm) = 3.125mm

Not to say that you remembering any of those numbers is important, but it might be helpful to understand. Especially why a number that goes up means more.

Got it?

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